Sputtering is the physical ejection of material from a target as a result of ion bombardment of the target. The ions are usually created by collisions between gas atoms and electrons in a glow discharge. The ions are accelerated into the target cathode by an electric field. A substrate is placed in a suitable location so that it intercepts a portion of the ejected atoms. Thus, a coating of target material is deposited on the surface of the substrate.
In an endeavor to attain increased deposition rates, magnetically enhanced targets have been used. For a planar magnetron, the cathode includes an array of permanent magnets arranged in a closed loop and mounted in a fixed position in relation to the flat target plate. Thus, the magnetic field is caused to travel in a closed loop, commonly referred to as a "race track," which establishes the path or region along which sputtering of the target material takes place. For a rectangular shaped target, the "race track" has a characteristic oval configuration wherein the erosion pattern in the target appears as a circular trough directly under the peak of the magnetic field lines. In a magnetron cathode, a magnetic field confines the glow discharge plasma and increases the path length of the electrons moving under the influence of the electric field. This results in an increase in the gas atom-electron collision probability. This leads to a much higher sputtering rate than that obtained without the use of magnetic confinement. Further, the sputtering process can be accomplished at a much lower gas pressure.
Heat generated during sputtering must be removed to insure adequate performance of the planar magnetron, but effective heat removal is often difficult to achieve in conventional planar magnetrons. This problem stems in part from the design of planar magnetrons wherein the cathode comprises a basic cathode body (or core) typically having one or two channels each of which is flanked by the sides of the core. Each channel houses permanent magnets that are connected by a magnet yoke. A planar target is positioned in proximity to the poles of the permanent magnets so that the target covers a substantial part of the channel. See, for example, Aichert et al., U.S. Pat. No. 4,572,776, issued Feb. 25, 1986. By positioning the magnets in the channel of the core, the core effectively confines most of the magnetic field to regions above the target. Unfortunately, with this design effective heat removal remains a problem.
Another problem associated with conventional planar magnetrons is low target utilization. Typically, with current large scale planar magnetrons, only about 28% of the target material is used before the entire target plate must be replaced. This is due to the fact that in present systems target material is sputtered primarily from a narrow region near the center of the "race track." This creates erosion patterns having pronounced grooves that would eventually penetrate through the target material into the support structure if the target material is not replaced in time.